Belt grinding machine

ABSTRACT

A belt grinding machine which is equipped with an elastically flexible load beam extending over the entire operative length of an endless grinding belt for applying the necessary grinding pressure to the belt. The effective pressure-applying surface of said beam is suspended for elevational control from support elements distributed at intervals along the length of the beam and attached to an elevationally movable carrier preferably in the form of transverse beams mounted on push rods operable by a pressure medium. The push rods are associated with constant delay mechanism adjustable to the difference in time between the instant of response of a sensing element to the passage of the leading end of a workpiece and the instant the grinding action begins, on the one hand, and between the instant of response of a sensing element to the passage of the trailing end of the workpiece and the instant the grinding action terminates, on the other hand.

United States Patent [151 3,694,966 Heesemann 1 Oct. 3, 1972 [541 BELT GRINDING MACHINE [72] Inventor: Karl Heesemann, 54, Friedenstrasse, Prlma'y f z z g i g Kelly Rehme via Bad Oeynhausen, Ger- Attorney ar Va ml many 57 ABSTRACT [22] Flled' @1970 A belt grinding machine which is equipped with an [21] Appl. No.: 87,655 elastically flexible load beam extending over the entire operative length of an endless grinding belt for apply- Related Application Data ing the necessary grinding pressure to the belt. The ef- [63] Continuation of Ser. No. 684,867, Nov. 21, festive P pp y surface of said beam is 1967,abandoned. suspended for elevanonal control from support elements distributed at intervals along the length of the 52 U.S. Cl ..51/138, 51/141 beam and attached to an elevationally movable carrier 51 Int. Cl. ..B24b 21/08 Preferably in the form of transverse beams mounted 581 Field of Search.....S1/l35 R, 141, 148, 137-139 on Push rods Perab1e by a N medlum- The push rods are associated with constant delay [56] References Cited mechanism adjustable to the difference in time between the instant of response of a sensing element UNITED STATES PATENTS to the passage of the leading end of a workpiece and the instant the grinding action begins, on the one Heesemann hand, an between the ns ant of response o a Sens ng 3,22 I Heesemann element to the p g of the trailing end of the work Keyes piece and the instant the g i g action terminates 3,374,583 3/1968 Jacobi ..5l/l4l on the other hand. 3,400,493 9/1968 Keyes ..51/138 3,408,775 11/1968 Rutt ..5 l/l4l X 13 Claims, 6 Drawing Figures PATENTED "UT 3 I972 SHEET 1 [1F 4 Fig.7

Invenlor:

KARL HE ESEMANN PATENTEUUBT 3 I972 SHEET 2 BF 4 Fig. 2

'IIA IIIIIVIA Inventor:

KAQL. HEESEMANN Inventor.

KARL HEESEIIANN A TTYJ'.

PATENTEUum 3 m2 SHEET Q UF 4 5&5

Fig.6

Inventor:

KAR L HEEJEMANN AT T7 BELT GRINDING MACHINE This is a continuation application of my copending application Ser. No. 684,867 filed Nov. 21, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a belt grinding machine equipped with an elastically flexible load beam extending over the entire operative length of an endless grinding belt for applying the necessary grinding pressure to the belt, the effective pressure-applying surface of said beam being suspended for elevational control from support elements distributed at intervals along the length of the beam and attached to elevationally movable carrier means preferably in the form of transverse beams mounted on push rod means operable by a pressure medium.

SUMMARY OF THE INVENTION In such belt grinding machines, particularly for grinding workpieces faced with thin veneers or coated with thin films of lacquer it is an object of the present invention to prevent the facing or coating at the leading and trailing edges as well as at the side edges of the workpiece from being cut through.

Another object of the invention is the provision of a belt grinding machine in which the effective pressureapplying surface is automatically controlled to conform with the contours of the workpiece.

In a belt grinding machine of the specified kind said objects are attained by the present invention in that the push rod means are associated with constant delay means adjustable to the difierence in time between the instant of response of a sensing means to the passage of the leading end of a workpiece and the instant the grinding action begins, on the one hand, and between the instant of response of a sensing means to the passage of the trailing end of the workpiece and the instant the grinding action terminates, on the other hand.

In a preferred embodiment of the invention a pawl is associated with the push rod means, for delaying the transmission of the movement of the push rod means or the operation of the actuating means interposed between the push rod means and the effective pressureapplying surface of the load beam after said sensing means have responded. To this end the preferably spring-loaded pawl may be operable by cam means, projections and the like, attached to the push rod means in such a manner that the interposed actuating means remain locked for a given constant period of delay after the response of the sensing means, to be released at the end of said period of delay to activate the flexible pressure-applying surface of the load beam and to lower the effective surface of the load beam into pressure-applying position.

Moreover, for improving the continuous adjustability of the effective pressure-applying surface of the load beam a preferred embodiment of the invention provides for the operational co-operation of neighboring push rods in such a way that push rods not directly controlled by the workpiece are entrained for partial operation by the push rods that are directly controlled by the workpiece.

In a belt grinding machine as proposed by the invention only that part of the pressure-applying surface of the load beam automatically takes effect which in size and shape conforms with the size and shape of the surface of the workpiece that is to be ground.

BRIEF DESCRIPTION OF THE DRAWINGS One embodiment of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is a front elevational view of a belt grinding machine showing a load beam and control means according to the invention;

FIG. 2 is a cross section of the load beam and of one of the control means, the section being taken on the line I-I ofFIG. 1;

FIG. 3 is a top plan view of the load beam showing the relatively offset disposition of the cylinders operable by a pressure medium for raising and lowering the push rods;

FIG. 4 is a top plan view of the disposition of the sensing and control means on the load beam;

FIG. 5 is an axial section of a piston chamber and push rod associated with the load beam, and

FIG. 6 is a top plan view of the upper portion of the lever mechanism in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the illustrated belt grinding machine which comprises an underframe 10 through which workpieces 11 that are to be ground are conveyed on a conveyor belt 13 or the like transversely across the direction of travel of a grinding belt 12, there is provided a flexible load beam 14 extending along the entire working length of the grinding belt. The load beam comprises U-section support members 15 and 16 interconnected by a baseplate 17 of the same length. Interposed between this baseplate 17 and an elastic steel plate 19 suspended from tierods 18a and 18b is a flexible tube 20 for the reception of a pressure medium, preferably air, for generating resilient pressure.

Adhesively affixed to the underside of the elastic steel plate 19 is a pad 21 made of felt, sponge rubber or the like of which the underface is covered by a loosely applied sliding mat 22. The sliding mat 22 is fastened to one end of the load beam 14 itself whereas at the other end of the beam 14 it is attached to tensioning means not shown in the drawings. The load beam extends above the endless grinding belt 12 of the belt grinding machine.

The grinding belt 12 which is likewise tensioned travels under the sliding mat 22 in continuous contact therewith. The entire pressure-applying surface of the beam 14 is shown in raised, i.e. in inoperative position.

The tiebolts 18a and 18b are held at their upper ends by counternuts 23, 24 in holding levers 25, 26.

Affixed to the front of the load beam 14 in the belt grinding machine is an elevationally adjustable supporting rail 27 extending across the entire width of the machine. Attached at equidistant intervals to the supporting rail 27 are angle brackets 28 which carry sensing levers 29 fulcrumed on pivot pins 30, each sensing lever carrying a roller 31 at its bottom end. Attached to each sensing lever 29 at a point between its pivot pin 30 and the roller 31 is a compression spring 32 which bears against an abutment fonned on the angle bracket 28. Above its pivot pin 30 the sensing lever 29 is formed with a cam edge 29a which cooperates with a ball bearing 33 mounted on a lever arm 35 fulcrumed on a pivot pin 34. The free end of the lever arm 35 carries a screw stop 36 which operates the actuating rod 37 of a three-way valve 38,

Moreover, between the roller 31 and the pivot pin 30 each sensing lever 29 is provided with an entraining arm 39 which is screwed to the sensing lever, and which with its free horizontal ends, provided with recesses 39a, extends across the two neighboring sensing levers 29, in front of the sensing levers when viewed in the direction of feed of the workpiece, without actually touching the same. The entraining arms 39 of neighboring sensing levers 29 are elevationally offset to prevent them from obstructing each other. The entraining arms of the outer sensing levers 29 have only an inwardly projecting end (cf. FIGS. 2 and 4).

In the illustrated embodiment pipe means comprising a pressure reducing valve and an accumulator admit compressed air at constant pressure directly via a connection 40 to the upper face of a piston 41 which is slidable in a chamber 42 formed by a cylinder 46 and a baseplate 47, the latter containing a passage 48 and fitted with a connection 49 for a flexible tube (cf. FIG.

Likewise through pipe means comprising a pressure reducing valve, accumulator, three-way valve 38 and a throttle valve 50 and buffering vessel 51 temporary counter-pressure is applied via connection 49 (cf. FIG. 5) to the underside of the piston 41 through a pipe 52 and released through an exhaust 53. The throttle valve 50 comprises a tapered body 500 including a helically threaded shaft 50b rotatably mounted in housing 50c to move towards and away from the valve seat. The throttle valve is controllable by a lever 54 attached to the end of shaft 501;. The lever 54 has a vertical slot 540 or the like at its free end engaged by a pin 65 on a rail 66. The rail 66 is held in a guide rail 67 which is longitudinally movable by a handwheel 68 and a lead screw 69, to adjust the angular position of the levers 54 of the throttle valves 50 and thereby centrally to control the rate of air flow through all of the throttle valves.

The piston 41 and its push rod 43 which is fitted with a cam 44 are axially movably guided in a cover 45 (cf. FIGS. 2 and 5).

The free end of the push rod 43 co-operates with the roller 55 of a control lever 56 which is fast on a rotatably mounted shaft 57. The same shaft also carries the holding levers 25 and 26 which retain the elastic steel plate 19 by means of the two tiebolts 18a and 18b (cf. FIG. 2).

When the pressure-applying face of the load beam 14 is in its raised inoperative position the roller 55 on control lever 56 bears on the projecting upper end of the push rod 43 (cf. FIG. 5).

Directly below the free end of the control lever 56 is a roller 60 on a pawl 61 fulcrumed on a pivot 61a and held in position by a compression spring 63 interposed between the pawl and an abutment arm 62 affixed to the cover 45. When the piston 41 descends as a result of the counterpressure falling to zero when the control means are actuated and the constant grinding pressure introduced through the tube connection 40 takes effect, a ramp 440 on the cam 44 attached to the push rod 43, after descending a predetermined distance,

rides up against a second roller 64 and thereby deflects the pawl 61 together with the roller against the counterpressure of the compression spring 63. This releases the control lever 56 permitting it to be tilted by the elastic pressure in the tube 20 which is transmitted thereto by the tiebolts 18a and 18b, the holding levers 25, 26 and the shaft 57.

Since the push rod 43 must first descend a given distance before the control lever 56 can yield to the downward pull of the tiebolts 18a and 18b there is a period of delay between the time of operation of the sensing means by the workpiece 11 and the time the pressure-applying face of the load beam I4 is released for operation.

The machine illustrated in FIGS. 1 to 6 operates as follows:

The workpiece 11 which is fed into the machine for grinding pushes the roller 31, which is adjusted to project slightly below the top of the workpiece, in the direction toward the load beam 14. The roller 31 is deflected along an arcuate path about the pivot pin 30 of the sensing lever 29 against the counter-pressure of the spring 32. The cam edge 290 at the upper end of the sensing lever 29 therefore rolls across the bottom of the ball bearing 33, deflecting the latter and hence the free end of the lever arm 35 upwards about the pivot pin 34. The adjustable screw stop 36 which, at the free end of the lever can be set to compensate working tolerations, thrusts the actuating rod 37 into the three-way valve 38 which closes the admission pipe 52' for compressed air and opens the exhaust 53.

The events triggered by the entering workpiece ll affect all those pistons 41 which are directly or indirectly controlled by the sensing levers 29 deflected by the infeeding workpiece. The outwardly extending ends of the entraining arms 39 on the outer sensing levers 29 entrain the neighboring sensing levers 29 which are not directly deflected by the workpiece II and these levers are therefore likewise deflected about their respective pivot pins 30. However, the angular deflection of these levers is less than that of the levers 29 which are directly deflected by the workpiece because the entraining arms 39 are formed with the recesses 390 at their ends. The angular deflections of neighboring sensing levers 29 by the co-operating entraining arms 39 sequentially lessen towards each side of the workpiece 11 and each lever on either side of the workpiece l l deflects the outwardly neighboring lever 29 through an angle that is slightly smaller than its own angle of deflection (cf. FIG. 4). Consequently the pressure-applying face of the load beam 14 will generate grinding pressure at all those points where the workpiece 11 moves underneath the beam 14. On each side the pres sure-applying face of the load beam 14 pressure is not applied to the flexible load beam at points spaced from the edges of the work-piece; therefore, the beam curves upwardly away to a normal position at a level spaced a given distance away from the top of the workpiece.

According to possible differences in the feed speed of the workpiece 11 the delay between the instant of deflection of the sensing lever 29 by the workpiece 11 and the application or relaxation of pressure by the load beam must be increased or reduced. This is done with the aid of the throttle valve 50 which operates in both directions of flow.

The throttle valve 50 is incorporated in that pipe system which supplies the compressed air to the underside of the piston 41. If according to the feed speed of the work the cross section of flow through the throttle valve is increased or reduced, the resultant delay will vary accordingly and can be adjusted to the required period.

It transpires that the leading edge of the workpiece travels a longer distance from the instant of making contact with the roller 31 to the instant the grinding operation begins than the trailing end of the workpiece from the instant of releasing the roller 31 to the instant the grinding operation terminates. This means that the period of delay at the beginning of the grinding operation must be longer than the period of delay at the termination of the grinding operation. Conveniently allowance can be made for this necessity by providing for additional delay before the beginning of the grinding operation by virtue of the cam 44 on the push rod 43 not deflecting the pawl 61 retaining the control lever 56 until the push rod 43 has already completed part of its descent (embodiment shown in FIGS. 5 and 6).

When the trailing edge of the workpiece ll releases the roller 31 the compression spring 32 at once returns the sensing lever 29 into its normal position. This causes the cam edge 290 on the sensing lever 29 simultaneously to lower the roller bearing 33 and the free end of the lever arm 35. The adjustable screw stop 36 releases the actuating rod 37 of the three-way valve 38. Hence the exhaust 53 is closed and at the same time the admission pipe 52 opened. The compressed air entering the inner chamber 42 below the piston 41 causes the pressure-applying face of the load beam 14 to be raised into inoperative position.

The proposed arrangement of a load beam ensures that workpieces fed into the machine will be evenly and fully ground throughout their surface without risk of the facing at the edges being cut through, irrespective of the size and shape of the workpiece and the particular position in which the workpiece enters the machine.

Control of the pressure-applying surface of the load beam so that it conforms with the workpiece could also be effected by means other than those herein described, for instance by photoelectric sensors or via magnetic valves.

It is also within the scope of the invention to embody the delay means in a variably adjustable device, such as a throttle valve or the like which is adjusted in dependence upon the speed of travel of the workpiece and also to provide a constant device such as a locking pawl or the like which operates in accordance with the difference in time between response to the end of the entering workpiece and the end of the grinding operation.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

I claim:

1. A grinding machine for grinding the surfaces of workpieces comprising a. an endless grinding belt,

b. means for conveying workpieces along a path towards-and adjacent to the operative length of said belt, said path being transverse to the direction of movement of said grinding belt,

c. means for releasably applying grinding pressure to the operative length of said belt,

. workpiece sensing means for detecting the leading and trailing edges of workpieces on said conveying means at a point along said path upstream of said grinding belt and for activating said pressure applying means when a leading edge is detected and deactivating said pressure applying means when a trailing edge is detected, and

e. delay means for delaying the application and release of pressure by said pressure applying means following activation and deactivation respectively of said pressure applying means, said delay means providing a first delay period from the time said sensing means detects the leading edge of said workpiece to the time said leading edge has advanced to a position adjacent said belt before pressure is applied to said belt and providing a second delay period from the time said sensing means detects the trailing edge of said workpieces to the time said trailing edge has advanced to a position adjacent said belt before pressure is released from said belt, said first delay period being longer than said second delay period,

f. whereby the surfaces of the workpieces may be ground without grinding down or damaging the edges of the workpieces.

2. A grinding machine as claimed in claim 1 wherein a. said pressure applying means comprises an elongated elastic plate arranged adjacent to the operative length of the belt, means biasing said plate towards said belt to apply pressure to said plate, and a plurality of retraction means, each for withdrawing pressure applied by a different portion of the length of said elastic plate,

b. said sensing means comprises a plurality of individual sensing elements arranged across said path upstream from said grinding belt, each for releasing one of said retraction means when detecting a workpiece and for actuating said retraction means when not detecting a workpiece, and

c. said delay means comprises a first and a second delay device operatively associated with each of said retraction means and the sensing element for controlling it, said first delay device being adapted to retard release of said retraction means and the application of pressure to said belt and said second delay device being adapted to retard actuation of said retraction means to withdraw application of pressure to said belt,

. whereby full pressure will be applied to only substantially that portion of the operative length of the grinding belt corresponding to the width of the workpiece and to the number and location of the sensing devices operated by the workpiece.

3. A grinding machine as claimed in claim 2 wherein each retraction means comprises a piston-cylinder arrangement operable by a pressure medium.

4. A grinding machine as claimed in claim 3 further comprising valve means operatively connected to each sensing element for controlling the application and release of said pressure medium to each piston-cylinder arrangement.

5. A grinding machine as claimed in claim 2 wherein said sensing elements comprise pivotally mounted sensing levers and entraining means for coupling neighboring sensing levers such that sensing levers directly deflected by contact with the workpiece operate to deflect adjacent sensing levers to a lesser extent whereby portions of said elastic plate beyond the sides of the workpiece apply a lesser pressure to the operative length of said belt.

6. A grinding machine as claimed in claim 5 wherein said entraining means comprises a plurality of entraining arms, one arm being attached to each of said sensing levers and extending transversely therefrom a sufficient distance to engage and deflect adjacent entraining arms.

7. A grinding machine as claimed in claim 3 wherein said piston-cylinder arrangement further comprises a push rod connected to said piston and mechanical means releasably connecting said push rod with a portion of said elastic plate and said first delay device comprises means to retard movement of said mechanical means in the direction to cause said portion of said elastic plate to apply pressure to said belt for a predetermined time after said push rod is moved in said direction.

8. A grinding machine as claimed in claim 7 wherein said means to retard movement of said mechanical means comprises a pawl and cam means on said push rod for acting on said pawl as said push rod moves in the direction to cause pressure to be applied to said belt.

9. A grinding machine as claimed in claim 8 further comprising spring means biasing said pawl towards said cam means.

10. A grinding machine as claimed in claim 3 further comprising valve means operatively connected to said sensing means for controlling said pressure medium flowing to and from said piston-cylinder arrangement and wherein said second delay device comprises throttle valve means for controlling the rate of flow of said pressure medium to the piston in said piston-cylinder arrangement to withdraw a portion of said elastic plate whereby said elastic plate may be retained in a position applying pressure to said belt for a predetermined time after the trailing edge of said workpiece has passed said sensing device.

11. A grinding machine as claimed in claim 10 wherein said throttle valve means comprises means for adjusting the cross-section of How through said throttle valve means whereby the period of delay after the trailing edge of the workpiece passes said sensing means and before the elastic plate is moved away from said belt may be controlled.

12. In a grinding machine for grinding the surfaces of workpieces and including means for conveying workpieces along a path past and in contact with the operative length of an endless grinding belt, the improvement comprising a. a plurality of sensing means extending transversely across said path upstream from said grinding belt to detect the presence of each workpiece on said conveyor means as it approaches said grinding belt,

b. a plurality of pressure applying means arranged to press said grinding belt against workpieces while being conveyed past said belt,

c. a plurality of means for moving said pressure applying means towards and away from said belt, said moving means being responsive to said sensing means such that detection of the presence of a workpiece by said sensing means causes said pressure applying means to press said belt against said workpiece and detection of no workpiece causes said pressure applying means to be withdrawn, and

d. delay means to delay movement of said pressure applying means towards and away from said belt such that said belt is not pressed by said pressure applying means until grinding action begins on a workpiece, the leading edge of which was previously detected upstream by said sensing means, and pressure on said belt by said pressure applying means is not released until grinding action terminates on said workpiece, the trailing edge of which was previously detected upstream by said sensing means, said delay means comprising pawl means to retain said pressure applying means in withdrawn position and cam means on said moving means to release said pawl means after movement of said moving means a predetermined distance following detection of the leading edge of a workpiece by said sensing means e. whereby the surface of the workpiece may be ground without grinding down or damaging the edges of the workpieces.

13. In a grinding machine for grinding the surfaces of workpieces and including means for conveying workpieces along a path past and in contact with the operative length of an endless grinding belt, the improvement comprising a. a plurality of sensing means extending transversely across said path upstream from said grinding belt to detect the presence of each workpiece on said conveyor means as it approaches said grinding belt,

. a plurality of pressure applying means arranged to press said grinding belt against workpieces while being conveyed past said belt,

c. a plurality of means for moving said pressure applying means towards and away from said belt, said moving means being responsive to said sensing means such that detection of the presence of a workpiece by said sensing means causes said pressure applying means to press said belt against said workpiece and detection of no workpiece causes said pressure applying means to be withdrawn, and

d. delay means to delay movement of said pressure applying means towards and away from said belt such that said belt is not pressed by said pressure applying means until grinding action begins on a workpiece, the leading edge of which was previously detected upstream by said sensing means, and pressure on said belt by said pressure applying means is not released until grinding action terminates on said workpiece, the trailing edge of which was previously detected upstream by said sensing means,

e. said moving means comprising fluid pressure operated means and said delay means comprising throttle valve means responsive to said sensing means for controlling the flow of pressure fluid to and from said fluid pressure operated means such said sensing means f. whereby the surface of the workpiece may be ground without grinding down or damaging the edges of the workpieces. 

1. A grinding machine for grinding the surfaces of workpieces comprising a. an endless grinding belt, b. means for conveying workpieces along a path towards and adjacent to the operative length of said belt, said path being transverse to the direction of movement of said grinding belt, c. means for releasably applying grinding pressure to the operative length of said belt, d. workpiece sensing means for detecting the leading and trailing edges of workpieces on said conveying means at a point along said path upstream of said grinding belt and for activating said pressure applying means when a leading edge is detected and deactivating said pressure applying means when a trailing edge is detected, and e. delay means for delaying the application and release of pressure by said pressure applying means following activation and deactivation respectively of said pressure applying means, said delay means providing a first delay period from the time said sensing means detects the leading edge of said workpiece to the time said leading edge has advanced to a position adjacent said belt before pressure is applied to said belt and providing a second delay period from the time said sensing means detects the trailing edge of said workpieces to the time said trailing edge has advanced to a position adjacent said belt before pressure is released from said belt, said first delay period being longer than said second delay period, f. whereby the surfaces of the workpieces may be ground without grinding down or damaging the edges of the workpieces.
 2. A grinding machine as claimed in claim 1 wherein a. said pressure applying means comprises an elongated elastic plate arranged adjacent to the operative length of the belt, means biasing said plate towards said belt to apply pressure to said plate, and a plurality of retraction means, each for withdrawing pressure applied by a different portion of the length of said elastic plate, b. said sensing means comprises a plurality of individual sensing elements arranged across said path upstream from said grinding belt, each for Releasing one of said retraction means when detecting a workpiece and for actuating said retraction means when not detecting a workpiece, and c. said delay means comprises a first and a second delay device operatively associated with each of said retraction means and the sensing element for controlling it, said first delay device being adapted to retard release of said retraction means and the application of pressure to said belt and said second delay device being adapted to retard actuation of said retraction means to withdraw application of pressure to said belt, d. whereby full pressure will be applied to only substantially that portion of the operative length of the grinding belt corresponding to the width of the workpiece and to the number and location of the sensing devices operated by the workpiece.
 3. A grinding machine as claimed in claim 2 wherein each retraction means comprises a piston-cylinder arrangement operable by a pressure medium.
 4. A grinding machine as claimed in claim 3 further comprising valve means operatively connected to each sensing element for controlling the application and release of said pressure medium to each piston-cylinder arrangement.
 5. A grinding machine as claimed in claim 2 wherein said sensing elements comprise pivotally mounted sensing levers and entraining means for coupling neighboring sensing levers such that sensing levers directly deflected by contact with the workpiece operate to deflect adjacent sensing levers to a lesser extent whereby portions of said elastic plate beyond the sides of the workpiece apply a lesser pressure to the operative length of said belt.
 6. A grinding machine as claimed in claim 5 wherein said entraining means comprises a plurality of entraining arms, one arm being attached to each of said sensing levers and extending transversely therefrom a sufficient distance to engage and deflect adjacent entraining arms.
 7. A grinding machine as claimed in claim 3 wherein said piston-cylinder arrangement further comprises a push rod connected to said piston and mechanical means releasably connecting said push rod with a portion of said elastic plate and said first delay device comprises means to retard movement of said mechanical means in the direction to cause said portion of said elastic plate to apply pressure to said belt for a predetermined time after said push rod is moved in said direction.
 8. A grinding machine as claimed in claim 7 wherein said means to retard movement of said mechanical means comprises a pawl and cam means on said push rod for acting on said pawl as said push rod moves in the direction to cause pressure to be applied to said belt.
 9. A grinding machine as claimed in claim 8 further comprising spring means biasing said pawl towards said cam means.
 10. A grinding machine as claimed in claim 3 further comprising valve means operatively connected to said sensing means for controlling said pressure medium flowing to and from said piston-cylinder arrangement and wherein said second delay device comprises throttle valve means for controlling the rate of flow of said pressure medium to the piston in said piston-cylinder arrangement to withdraw a portion of said elastic plate whereby said elastic plate may be retained in a position applying pressure to said belt for a predetermined time after the trailing edge of said workpiece has passed said sensing device.
 11. A grinding machine as claimed in claim 10 wherein said throttle valve means comprises means for adjusting the cross-section of flow through said throttle valve means whereby the period of delay after the trailing edge of the workpiece passes said sensing means and before the elastic plate is moved away from said belt may be controlled.
 12. In a grinding machine for grinding the surfaces of workpieces and including means for conveying workpieces along a path past and in contact with the operative length of an endless grinding belt, the improvement comprising a. a plurality of sensing meanS extending transversely across said path upstream from said grinding belt to detect the presence of each workpiece on said conveyor means as it approaches said grinding belt, b. a plurality of pressure applying means arranged to press said grinding belt against workpieces while being conveyed past said belt, c. a plurality of means for moving said pressure applying means towards and away from said belt, said moving means being responsive to said sensing means such that detection of the presence of a workpiece by said sensing means causes said pressure applying means to press said belt against said workpiece and detection of no workpiece causes said pressure applying means to be withdrawn, and d. delay means to delay movement of said pressure applying means towards and away from said belt such that said belt is not pressed by said pressure applying means until grinding action begins on a workpiece, the leading edge of which was previously detected upstream by said sensing means, and pressure on said belt by said pressure applying means is not released until grinding action terminates on said workpiece, the trailing edge of which was previously detected upstream by said sensing means, said delay means comprising pawl means to retain said pressure applying means in withdrawn position and cam means on said moving means to release said pawl means after movement of said moving means a predetermined distance following detection of the leading edge of a workpiece by said sensing means e. whereby the surface of the workpiece may be ground without grinding down or damaging the edges of the workpieces.
 13. In a grinding machine for grinding the surfaces of workpieces and including means for conveying workpieces along a path past and in contact with the operative length of an endless grinding belt, the improvement comprising a. a plurality of sensing means extending transversely across said path upstream from said grinding belt to detect the presence of each workpiece on said conveyor means as it approaches said grinding belt, b. a plurality of pressure applying means arranged to press said grinding belt against workpieces while being conveyed past said belt, c. a plurality of means for moving said pressure applying means towards and away from said belt, said moving means being responsive to said sensing means such that detection of the presence of a workpiece by said sensing means causes said pressure applying means to press said belt against said workpiece and detection of no workpiece causes said pressure applying means to be withdrawn, and d. delay means to delay movement of said pressure applying means towards and away from said belt such that said belt is not pressed by said pressure applying means until grinding action begins on a workpiece, the leading edge of which was previously detected upstream by said sensing means, and pressure on said belt by said pressure applying means is not released until grinding action terminates on said workpiece, the trailing edge of which was previously detected upstream by said sensing means, e. said moving means comprising fluid pressure operated means and said delay means comprising throttle valve means responsive to said sensing means for controlling the flow of pressure fluid to and from said fluid pressure operated means such that said fluid pressure means is not moved and said pressure applying means is retained against said belt until predetermined volumes of fluid to and from said fluid pressure operated means have passed through said throttle valve means following detection of the trailing edge of a workpiece by said sensing means f. whereby the surface of the workpiece may be ground without grinding down or damaging the edges of the workpieces. 